The present applicant has undertaken work relating to the architecture of a propulsion unit with at least two distributed fans as described in the document US-A1-2008/0098719. The objective of this was to seek optimisation of the propulsion efficiency by means of a high by-pass ratio while keeping an acceptable ground clearance and fans of small size.
The art prior to the present application also includes U.S. Pat. Nos. 2,875,599, 4,871,130, and 1,916,442.
An outline diagram of a propulsion unit 1 is shown in FIGS. 1 and 2. A gas generator 3 comprises in a conventional fashion one or more compressors supplying air to a combustion chamber; the gases issuing from the chamber drive one or more turbines connected to the compressors and in this case a free turbine 5. The latter is rigidly connected to a power shaft 6 coaxial with the shaft of the gas generator 3. This power shaft 6 drives, by means of suitable bevel gears, two intermediate radial shafts 8 and 8′ disposed in particular at right angles with respect to the axis of the gas generator. The intermediate radial shafts each drive a fan shaft 9, 9′ offset from, and parallel to, the axis of the gas generator. The power is transmitted by means of first bevel gear units 2 and 2′ between the shaft 6 and the radial shafts 8 and 8′ and second bevel gears 4 and 4′ between the radial shafts 8, 8′ and the fan shafts 9 and 9′. The shafts of the fans 9 and 9′ each drive a fan 10, 10′ of which the axis is parallel to that of the gas generator. Such an arrangement makes it possible to achieve the objectives referred to above.
According to the prior art known to the applicant, the solution to the problem of transmission between the free turbine and the fan shafts consists in using, for each of the fans, simple bevel gears at 45°, i.e. a first set of gears with two gear wheels, one on the shaft of the free turbine and the other on the radial shaft, and a second set of bevel gears at 45° with two gear wheels connecting the radial shaft to the shaft of the fan.
This solution does not take into account the large movements that take place during operation between the fan modules and the module of the gas generator because of the thermal and mechanical loads on the assembly—there may be relative movements between the gas generator and the fan modules. Couplings between the shafts by means of flexible flanges forming flectors make it possible to reduce the level of stresses in the teeth of the gears through localised deformations in the case of angular misalignment. However, this solution remains insufficient since it is limited in amplitude and does not make it possible to effectively compensate for axial misalignments. In addition, this solution has the drawback of giving rise to high stresses in the flexible flange in the event of misalignment and these may cause problems of fatigue of the material.